The power plants from which large amounts of process steam are removed for industrial purposes are generally known. Relatively large amounts of steam are removed from such power plants for a prolonged or even the entire operating time. In various industrial applications for process steam, such as for example in paper factories, there is no return flow into the water/steam circuit of the power plant. Therefore, the amount of condensate and steam in the circuit has to be maintained by continuously supplying correspondingly large amounts of make-up water.
If only small amounts of make-up water are supplied to a power plant, or if this water is supplied only for a brief period, this make-up water is generally supplied directly to the steam condenser, for example by being sprayed over the tube bundles, where it is degassed in coolers which are present. The steam/gas mixture which is formed is extracted by bleed devices.
By contrast, if large amounts of make-up water are supplied to a power plant over a prolonged period or continuously, this make-up water is firstly degassed in a degassing system and is only then supplied to the condenser. Both degassing means and condenser are connected to bleed pumps which remove the steam/gas mixture from the circuit of the power plant. Compared to a power plant to which small amounts of make-up water are supplied, the demands imposed on the capacity of the bleed system are increased. These capacity demands are often determined by desired limits or ranges for the condenser pressure and for the oxygen content in the condensate which is taken out of the condenser for reuse for steam production. The lower these desired limits and the larger the amounts of steam and make-up water, the greater the demands imposed on the capacity of the extraction system.
FIG. 1 diagrammatically depicts part of a steam power plant with an example of a bleed system from the prior art, which removes the steam/gas mixture from a make-up water degassing means and a condenser. In this case, a suction arrangement is connected to the two units via two lines, each of the two lines having a diaphragm of predetermined aperture size, by means of which the suction capacity at the individual subsystems is predetermined. The size of these individual diaphragms and the ratio of the two diaphragm sizes, for a specific operating load of the power plant, that is to say a specific amount of steam to the condenser, and for a specific amount of make-up water, are such that an oxygen content in the condensate and a condenser pressure which are within the respectively desired ranges are established.
However, in power plants from which process steam is removed for industrial purposes, the amount of process steam removed and the supply of make-up water may vary considerably over time. At the same time, the current consumption and thus the amount of steam to the condenser may also fluctuate. However, if the suction capacities at the degassing system and condenser are predetermined by the diaphragm sizes, while the amounts of steam and make-up water vary, there is no guarantee that the overall system will be optimally set. For example, a predetermined distribution of the suction capacity to condenser and degassing means may lead to a very low oxygen content in the condensate, which is well below the desired limit, while, however, the bleeding of the condenser is insufficient, so that the condenser pressure rises. This reduces the condenser capacity and leads to associated losses in electrical output.
To avoid the risk of insufficient suction capacity, the total capacity of the bleed system may be sufficiently great to ensure that there is sufficient suction capacity for any possible current consumption and for any possible amount of make-up water supplied. However, at lower operating loads and with small amounts of make-up water, this would lead to excess capacity on the part of the bleed system and to unnecessary investment and operating costs.